This is a combination of 13 commits:
1. New signal for registering custom generators
2. New plugin: pelican.plugins.sitemap
3. pelican.plugins.sitemap: more settings
4. pelican.plugins.sitemap: translations are indexed
5. pelican.plugins.sitemap: added documentation
6. pelican.plugins.sitemap: added XML DTD & W3C dates
7. pelican.plugins.sitemap: removed a <changefreq> bug
8. the `get_generators` can now return a tuple
9. pelican.plugins.sitemap: cleaned the code
10. pelican.plugin.sitemap: settings changes
11. sitemap plugin: improved configuration & documentation
12. sitemap plugin: :set spell
13. sitemap plugin: removed useless whitespaces
